1. Field of the Invention
The present invention relates to control of a vibration type actuator which is referred to as an ultrasonic motor or the like, and excites vibration in an elastic member by applying a periodic signal to an electromechanical energy conversion element, thereby relatively driving the elastic member and a contact member which is in contact with the elastic member.
2. Related Background Art
Up to now, a proposal for reducing the power consumption of a vibration type actuator has been made in U.S. Pat. No. 6,100,654 and the like. According to the proposal, when the drive frequency of a vibration type motor is controlled, in a frequency region where a drive speed of the vibration type motor is low, a drive voltage is reduced or the pulse width of a pulse signal inputted to a drive circuit is narrowed, thereby improving the efficiency.
However, in a method of controlling the drive voltage or the pulse width according to a frequency region, which is proposed in U.S. Pat. No. 6,100,654 described above, there are the following problems.
1. With respect to the drive voltage or the pulse width of the pulse signal, which is set in the frequency region where the drive speed is low, the drive voltage or the pulse width of the pulse signal is set with a margin (so as not to stop driving) in the frequency region where the drive speed is low in view of a variation in performance of the vibration type actuator. Therefore, the power consumption cannot be sufficiently reduced.
2. The frequency and voltage of a periodic signal necessary to drive the vibration type actuator at target speed are changed according to a size of a load driven by the vibration type motor and use environments such as a temperature and a humidity. Therefore, when the drive voltage or the pulse width is set with a margin with respect to a variation in load and variations in use environments, the power consumption cannot be sufficiently reduced as described above.
3. According to an individual difference between parts (capacitor, coil, and the like) composing a circuit for generating the periodic signal applied to the vibration type motor and a variation in characteristic resulting from the use environments, the voltage (amplitude) of the periodic signal generated by the circuit varies. Therefore, it is hard to always perform driving with high efficiency.